Answer. 64 Kibibytes, otherwise stated as 64 * 2^10 bytes, otherwise stated as 65536 bytes.
This accomodates content files of up to approximately 600 Mebibytes in size.
As a reference, I've included a table that lists the size of certain content files and the approximate .share file that describes them:
| Original content file size | .share file size |
| 1MB | 0.078125 KB |
| 256MB | 20 KB |
| 512 MB | 40 KB |
| 1 GB | 80 KB |
| 2 GB | 160 KB |
| 4 GB | 320 KB |
These approximate results were calculated by dividing the original size of the file by the default piece size (2^18).
The result of thise division is the number of pieces there are in the original content file.
Since each piece of the file is hashed using the SHA1 algorithm, and the output of the SHA1 is 20 bytes long, the resulting file is 20 bytes * the number of pieces in the file.
While there are other forms of overhead associated with the .share file, the SHA1 hashes of the pieces is the most dominating factor in size.
For clarity
1 KB = 1 Kibibyte = 1024 bytes
1 MB = 1 Mibibyte = (1024)^2 bytes
1 GB = 1 Gibibyte = (1024)^3 bytes